The catalytic epoxidation of 2-cyclohexen-1-one over uncalcined layered double hydroxides using various solvents

Article abstract of DOI:10.1016/j.cattod.2014.02.031

The epoxidation reaction of an α,β-unsaturated ketone (2-cyclohexen-1-one), that is, an electron deficient C=C bond was performed over as-prepared and calcined layered double hydroxides (LDHs) of both the hydrotalcite- and the hydrocalumite type. It was found that the as-prepared LDHs always performed better than the calcined derivatives. Among them, the CaFe-LDH was the most active. The optimum reaction temperature and the most suitable solvent were also found after performing several set of reactions.

Full text of DOI:10.1016/j.cattod.2014.02.031

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Catalysis Today
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The catalytic epoxidation of 2-cyclohexen-1-one over uncalcined
layered double hydroxides using various solvents
M. Sipiczkia,g, A.A. Ádáma,g, T. Aniticsa,g, Z. Csendesb,g, G. Peintlerc,g, Á. Kukoveczd,e
,
Z. Kónyad,f, P. Siposa,g, I. Pálinkób,g,∗
a Department of Inorganic and Analytical Chemistry, University of Szeged, Szeged H-6720, Hungary
b Department of Organic Chemistry, University of Szeged, Szeged H-6720, Hungary
c Department of Physical Chemistry and Materials Science, University of Szeged, Szeged H-6720, Hungary
d MTA−SZTE “Lendület” Porous Nanocomposites Research Group, Rerrich B. tér 1, Szeged H-6720, Hungary
e Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B. tér 1, Szeged H-6720, Hungary
f
MTA−SZTE Reaction Kinetics and Surface Chemistry Research Group, Rerrich B. tér 1, Szeged H-6720, Hungary
g Material and Solution Structure Research Group, Institute of Chemistry, University of Szeged, Szeged H-6720, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
The epoxidation reaction of an ␣,␤-unsaturated ketone (2-cyclohexen-1-one), that is, an electron defi-
cient C=C bond was performed over as-prepared and calcined layered double hydroxides (LDHs) of both
the hydrotalcite- and the hydrocalumite type. It was found that the as-prepared LDHs always performed
better than the calcined derivatives. Among them, the CaFe-LDH was the most active. The optimum reac-
tion temperature and the most suitable solvent were also found after performing several set of reactions.
Received 30 September 2013
Received in revised form 4 February 2014
Accepted 6 February 2014
Available online xxx
Keywords:
© 2014 Elsevier B.V. All rights reserved.
Epoxidation of an ␣,␤-unsaturated ketone
Layered double hydroxides
Structural characterisation
Optimisation of the reaction conditions
1. Introduction
large amount of waste. One of the best ways to avoid these disad-
vantages is changing the stoichiometric process to a catalytic one,
Epoxidation is one of the most important reactions in organic
chemistry, since the epoxide ring may be opened in several ways
allowing the construction of complicated organic structures. Real-
life organic compounds mostly contain other functional groups
beside the easily epoxidisable olefinic double bond, like a carbonyl
group in conjugation with this functionality. The epoxidation of
electron-deficient carbon–carbon double bond of ␣,␤-unsaturated
ketones is of interest, because of the synthetic utility of the resulting
intermediates in the further functionalisation of ketones.
The most common procedure for the epoxidation is using hydro-
gen peroxide under strong alkaline conditions applying bases such
as NaOH, KOH, Na2CO3 or K2CO3. Aqueous hydrogen peroxide is an
ideal oxidant because it is both cheap and safe, yielding only water
as a co-product. However, the use of such strong bases is undesir-
able, because they are hazardous and lead to the production of a
if possible, using heterogeneous catalyst.
The epoxidation of olefins proceeds by nucleophilic attack of a
perhydroxyl anion on olefinic carbons [1]. In the same advanced
textbook [2], the mechanism of the reaction between H2O2 and
2-cylohexen-1-one (incidentally, these were our two reactants as
well) is detailed. It is demonstrated there that under basic condi-
tions (OH− was the example), the peroxide was deprotonated, and
then, conjugate addition took place. The product enolate is not sta-
ble, and this time the hydroxide ion is a good leaving group and
leaves with an E1cB elimination mechanism with the cleavage of
the weak O−O bond. Then, the remaining oxide ion, being a strong
nucleophile, makes the epoxide ring, and exclusively this product is
obtained, that is, the carbonyl group remains unchanged. Since the
layered double hydroxide-type (LDH-type) materials, being basic,
efficiently transform hydrogen peroxide into a perhydroxyl anion
on their surfaces [3], it may be expected that an LDH-catalysed sys-
tem can be developed for the epoxidation of olefins and even of
electron deficient olefins like 2-cyclohexen-1-one. Organic peroxi-
des without hydrogen to be deprotonated do not work this way,
however, they can be very effective in epoxidation, if the catalysts
∗
Corresponding author at: Corresponding author. Tel.: +36 62 544 288;
fax: +36 62 544 200.
E-mail address: palinko@chem.u-szeged.hu (I. Pálinkó).
http://dx.doi.org/10.1016/j.cattod.2014.02.031
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Sipiczki, et al., The catalytic epoxidation of 2-cyclohexen-1-one over uncalcined layered double
hydroxides using various solvents, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.031

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are noble metals (Ag, Au or both) supported on various oxide,
titanosilicate, etc. supports (for a recent review, see Ref. [4] and
for a very recent original work, see Ref. [5]).
Because of their relative ease of synthesis, LDHs represent
an inexpensive, a versatile, and a potentially recyclable source
of a variety of catalyst supports, catalyst precursors, or actual
catalysts [6–9]. As the name says, these materials have layered
structure, resembling that of brucite, a layered Mg(OH)2 consist-
ing of edge-sharing M(OH)6 octahedra. In a recent publication [10]
commissioned by the International Mineralogical Association, this
group is called the hydrotalcite supergroup consisting of eight sub-
groups of naturally occurring minerals.
In hydrotalcites, the Mg2+ ions are partially substituted by triva-
lent ions of, generally, similar radii. However, under extreme syn-
thesis conditions, this statement can be overruled, and for example,
Ba(II)Fe(III)-LDHs could be synthesised under hyperalkaline condi-
tions [11,12]. The generated positive charge of the layers is com-
pensated by anions that are located, together with water molecules,
in the interlayer region. The general formula of hydrotalcites is
[MII MIxII OH 2](An− ) × mH2O, where MII and MIII are the di- and
(
)
1−x
x/n
trivalent metal cations, and An− is the interlayer anion [13–16].
Although the octahedral arrangement is typical around the cations
in the supergroup, in a subgroup called hydrocalumites − the name
giving mineral has the formula of [Ca2Al(OH)6]A × nH2O − the cor-
rugated brucite-like main layers contain an ordered arrangement
of Ca2+ and Al3+ or other trivalent ions, seven- and six-coordinated,
respectively, in a fixed ratio of 2:1 [17].
Mixed metal oxides may be obtained by controlled thermal
decomposition of LDHs having large specific surface areas, basic
properties, a homogeneous and thermally stable dispersion of the
metal ion components, synergetic effects between the elements,
and the possibility of structure reconstruction under humid
conditions [18]. The thermal treatments at low temperature
lead to synergetic effects between the elements in spinel-like or
mixed oxide with strong basic sites. The presence of these sites
is advantageous in the epoxidation of unfunctionalised olefins;
nevertheless, reaction proceeds well if activating agents (nitriles,
amides) are also present [19]. However, the structural hydroxyl
groups of the uncalcined LDHs and the mixed hydroxide nature
of the layers − although the use of uncalcined, pristine LDH as
catalyst is significantly scarcer, but not unprecedented [20] − may
play important role in the selective epoxidation of multifunctional
compounds. Results obtained studying the selective epoxidation
of such a compound, the 2-cyclohexen-1-one, is described in the
followings.
Fig. 1. XRD patterns of the freshly prepared and air-dried LDHs, with (a) varying
Ca:Fe ratios and (b) varying ions with M(II):M(III) = 2:1 ratio. Reflections associated
with Ca(OH)2 side-product are indicated with *.
in the 3–60◦ 2ꢀ region with 4◦/min scanning rate on a Rigaku Mini-
flex II instrument using the CuK˛ radiation (ꢁ = 0.15418 nm). The
(003) reflection was used for calculating the interlayer spacings.
The accurate positions of the reflections were determined after
fitting Gaussian-type functions.
The morphologies of the samples were studied with a Hitachi
S-4700 scanning electron microscope (SEM) at various magnifica-
tions and a transmission electron microscopic (TEM) image taken
by a FEI TECNAI G220 X-TWIN microscope at 200 kV accelerating
voltage.
The X-ray absorption spectra (XAS) were measured at beamline
I511-3 at the MaxLab facility, Lund, Sweden. The station is based
on a superconductive undulator injection device connected to the
1.5 GeV MAX II storage ring. X-ray radiation in the 50–1500 eV
energy range can be obtained from this system.
2. Experimental
2.1. Materials and synthesis
The LDHs were prepared by the co-precipitation method. Solu-
tions of CaCl2, Al(NO3)3·9H2O, Mg(NO3)2·9H2O and FeCl3·6H2O (all
salts were purchased from Aldrich Chemic Co.) with M(II):M(III)
molar ratios ranging from 4 to 2 were transformed to LDH with
hot (ca. 80 ◦C) carbonate-free 3 M aqueous NaOH (Spektrum 3D, a.
r.) solution under vigorous stirring and N2 blanket. The precipitates
formed were rapidly filtered until air dry in a CO2-free atmosphere,
with the aid of a caustic resistant vacuum filter unit (Nalgene)
equipped with an appropriate membrane (Versapor, 0.45 ␮m). The
solid material was washed and filtered and the obtained crystals
were kept at room temperature in a desiccator over P2O5.
2.3. Reaction conditions and analysis
The pristine, as-prepared LDHs were tested in the epoxidation
of an electron-deficient C=C double bond of an ␣,␤-unsaturated
ketone, 2-cyclohexen-1-one (Aldrich Chemical Co.), that is, using
30 wt% aqueous hydrogen peroxide (VWR) as oxidant. The reactions
2.2. Characterisation methods
The key method for studying the structural features of the cata-
lysts was powder X-ray diffraction. The diffractograms were taken
Please cite this article in press as: M. Sipiczki, et al., The catalytic epoxidation of 2-cyclohexen-1-one over uncalcined layered double
hydroxides using various solvents, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.031

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3
Fig. 2. X-ray absorption spectra of the Ca3Fe-LDH sample performed at the (a) Ca2p, (b) Fe2p and (c) O1s absorption edges.
were performed in glass vials under mild reaction conditions
(vigorous stirring at 298 K for 2 h) applying various solvents
(methanol, ethanol, 2-methyl-2-propanol, acetone, formamide,
1,4-dioxane, cyclohexane, n-hexane). Temperature dependence, in
the 288 − 343 K range, was also examined. The general reaction
conditions were as follows: 1 mmol 2-cyclohexen-1-one, 4 mmol
H2O2, 2.5 mL solvent, 0.075 g catalyst.
[17]. Here, all the reflections typical of a LDH could be found
and there were no other ones. For the samples with higher
Ca(II)/Fe(III) ratios new reflections showed up, which could clearly
be assigned to a Ca(OH)2 phase. As described in our previous work
[21], this Ca(OH)2 phase stabilises the LDH phase; therefore, it is
advantageous in long-term applications, such as applying it as cat-
alyst.
The epoxidation of 2-cyclohexen-1-one was followed by gas
chromatography with a Hewlett-Packard 5890 Series II: instru-
ment equipped with 50-m-long HP-1 column and flame ionisation
detector. The internal standard technique was used for quantita-
tive analysis. The analysis conditions were varied to check whether
all the products formed are detected and a sample was checked
by 13C NMR spectroscopy. It was found that beside the unreacted
organic compound only the epoxide was formed. The proportion of
the remaining H2O2 at the end of the reaction was determined by
titration with KMnO4.
X-ray absorption spectra of the pristine Ca3Fe-LDH sample reg-
istered at the Ca2p, Fe2p and O1s edges are seen in Fig. 2.
Both the Ca and Fe XAS contain two intense peaks, since the
2p and the 2p1/2 energies are close to each other for both ele-
ments. The less intense peaks probably indicate separate phases
and/or imperfections in the crystal structure. We know from XRD
measurements that Ca(OH)2 is present as a minor but important
stabilising component beside the LDH. Since there is no sign of a
separate Fe-containing non-LDH phase, the splitting in the peak
at higher energy in the Fe X-ray absorption spectrum may be due
to imperfections, probably vacancies in the LDH crystal structure.
The O1s X-ray absorption spectrum has two pre-edge features indi-
cating that oxygen is present in various environments. Beside the
octahedral location in the LDH structure, the geometric environ-
ment in the Ca(OH)2 phase is probably different, just as at the
imperfections of the LDH crystal lattice.
3. Results and discussion
3.1. Structural features of the LDHs
SEM images displayed, indicate the layered structure for the
chosen samples (Fig. 3). This is fortunate, since arrangement at
the atomic level is not always reflected in the morphology of the
materials.
The typical reflections (003, 006) appearing on the diffrac-
tograms and most of the interlayer distances (calculated from the
positions of the first reflections) in Fig. 1 prove that our syntheses
were successful.
The basic shape of an LDH crystal is a hexagonal platelet. When
viewed by SEM, the LDH crystals appear as platelets indeed, but
The XRD traces revealed that the Ca(II)2M(III)-LDHs were only
phase-pure, coinciding with the observation of Rousselot et al.
Please cite this article in press as: M. Sipiczki, et al., The catalytic epoxidation of 2-cyclohexen-1-one over uncalcined layered double
hydroxides using various solvents, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.031

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Fig. 3. SEM images of (a) Ca2Al-, (b) Ca2Fe-, (c) Mg2Al- and (d) Mg2Fe-LDH.
the actual hexagonal shape is more clearly seen in the TEM image
(Fig. 4).
3.2. The catalytic reactions
The hexagonal platelets are the most common appearance of
an LDH with simple anions (halides, nitrate and carbonate), even
if the LDH is a rhombohedral crystal polytype. This is not a con-
tradiction, because the rhombohedral polytype refers to the layer
stacking sequence and the hexagonal platelets refer to overall crys-
tal growth.
The LDHs (CaFe-LDH, MgFe-LDH and CaAl-LDH) were used in
uncalcined forms, and for comparison, the reaction was performed
over TiO2 (P25), calcined CaFe-LDH as well as without catalyst.
The epoxidation of 2-cyclohexen-1-one takes place according to
the following equation (Scheme 1)
In the first set of experiments, the reaction time was 2 h and the
obtained conversions of cyclohexene-1-one were determined and
listed in Table 1. The reproducibility of the data was within 1%. It
is to be noted that H2O2 was not only consumed by the unsaturated
ketone, but also it underwent to appreciable decomposition (see,
columns 4 and 5 in Table 1).
Each uncalcined LDH was more active than the calcined CaFe-
LDH, clearly proving that the mixed hydroxide structure was
advantageous for the epoxidation of 2-cyclohexen-1-one. It is
worth noticing that the hydrocalumite-type LDHs resulted in better
conversions than the hydrotalcite-like layered materials. Varia-
tions in the Ca:Fe ratio had some minor effect on the activity
rendering Ca2Fe-LDH is the most active, affording 47% yield of the
2,3-epoxycyclohexanone. It is known that perfect hydrocalumite
structure only forms at this composition, pointing again at the
observation that the perfect mixed hydroxide layers suit best to
the reaction. However, as was discussed before, as far as stability is
Fig. 4. TEM image of Ca3Fe-LDH.
Scheme 1. The epoxidation reaction of 2-cyclohexen-1-one.
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hydroxides using various solvents, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.031

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Table 1
Conversion data of 2-cyclohexen-1-one in the epoxidation reaction with H2O2 over various LDHs and titania in methanol as solvent (composition: 1 mmol 2-cyclohexen-1-one,
4 mmol H2O2, 2.5 mL solvent, 0.075 g catalyst, reaction temperature: 298 K).
Catalyst
Conversion (%) after 120 min
Unconverted H2O2 (%)
H2O2 consumption (mmol)
H2O2 utilisation (mmol)
Without catalyst
TiO2 (P25)
Calcined Ca3Fe-LDH
Mg2Al-LDH
Mg2Fe-LDH
Ca2Al-LDH
Ca2Fe-LDH
Ca3Fe-LDH
Ca4Fe-LDH
<1
8
99.84
97.02
96.95
93.66
95.54
78.34
−
0.006
0.119
0.122
0.254
0.178
0.866
4.000
2.275
0.983
0.006
0.082
0.986
0.192
0.154
0.267
0.474
0.359
0.295
10
19
15
27
47
36
29
43.12
75.42
Table 2
Table 3
Conversion data of 2-cyclohexen-1-one in the epoxidation reaction with H2O2
over uncalcined Ca3Fe-LDH using methanol as solvent (composition: 1 mmol 2-
cyclohexen-1-one, 4 mmol H2O2, 2.5 mL solvent, 0.075 g catalyst).
Conversion data of 2-cyclohexen-1-one in the epoxidation reaction with H2O2
after 2 h over uncalcined Ca3Fe-LDH using various solvents (composition: 1 mmol
2-cyclohexen-1-one, 4 mmol H2O2, 2.5 mL solvent, 0.075 g catalyst, reaction tem-
perature: 298 K).
Temperature
Conversion (%) after 120 min
Solvent
Conversion (%)
288 K
27
298 K
313 K
328 K
343 K
Reaction time
5 min
30 min
1 h
36
39
26
29
2-Methyl-2-propanol
Ethanol
Methanol
<1
27
36
18
12
58
42
49
Acetone
Conversion (%) at 298 K
2
11
22
1,4-Dioxane
Formamide
Cyclohexene
n-Hexane
2 h
36
4 h
48
6 h
56
24 h
Catalyst
Ca3Fe-LDH (freshly prepared)
Reuse I
Reuse II
63
reactivity of the catalyst. The highest conversion was observed in
formamide; n-hexane and cyclohexene were better solvents than
methanol. In turn, methanol proved to be a better solvent for
this reaction than ethanol. The rest of the solvents, 2-methyl-2-
propanol, acetone and 1,4-dioxane performed poorly (Table 3).
The excellent activity in the presence of formamide was most
likely due to the delamination of the layers (the 003 reflection typ-
ical of the LDH structure disappeared from the diffractogram of
the material treated in formamide) making the basic sites signif-
icantly more accessible [23]. If the solvent is alcohol, lengthening
the carbon chain results in a dramatic decline in conversion. The
explanation of the poor conversion rate in 1,4-dioxane may lie in
the effect of the solvent on the LDH. It is similar to that of heat
treatment, in the sense that both destroy the structure, as Fig. 5
attests.
Conversion (%) after 120 min at 298 K
36
36
36
concerned the Ca:Fe = 3:1 ratio is the best [21], and since the activ-
ity was just slightly lower than with Ca2Fe-LDH, the Ca3Fe-LDH
was used for temperature-, time- and solvent-dependence studies.
The effect of reaction temperature on the conversion of the unsat-
urated ketone was also examined. Here, methanol was the solvent
of choice, and the results are summarised in Table 2.
The highest conversion of 2-cyclohexen-1-one was observed
when the reaction was performed at low temperatures. The
increase in reaction temperature over 40 ◦C resulted in a slight
decrease in the conversion, indicating that proportion of decom-
posed H2O2 increased with rising temperature. Epoxidation at 40 ◦C
was the most suitable from the standpoints of hydrogen perox-
ide efficiency and the reaction rate, in good agreement with the
observations of Yamaguchi et al. [22].
To achieve the maximum conversion, a time-dependence study
was performed. The results are summarised in Table 2. The epox-
idation of 2-cyclohexen-1-one carried out with the as-prepared
Ca3Fe-LDH in methanol solvent at 25 ◦C led to 36% conversion in 2 h
and 63% in 24 h with 2,3-epoxycyclohexanone as the only product.
The time-dependence data were fitted based on first-order kinetics.
As a result, the experiments strictly follow the first-order kinetics
with k = 0.0068 0.0003 min−1
.
The use of a solid LDH makes the workup procedure very simple.
The catalyst could easily be separated from the reaction mixture
by a simple filtration. The recovered catalyst was then reused in
the same reaction only by rinsing it with the solvent of the reac-
tion (Table 2). The spent catalyst performed very well, it gave the
epoxyketone with the same conversion (under the same condi-
tions) as in the first run. The conversion did not decrease even when
the catalyst was recycled twice.
A set of experiments were run with altering the solvent, since
occasionally, it has dramatic effect on the conversion and thus the
Fig. 5. The effect of heat treatment (500 ◦C) and treatment with 1,4-dioxane on
Ca3Fe-LDH.
Please cite this article in press as: M. Sipiczki, et al., The catalytic epoxidation of 2-cyclohexen-1-one over uncalcined layered double
hydroxides using various solvents, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.031

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This research was financed by the OTKA NK 106234 and
the TÁMOP 4.2.2.A-11/1/KONV-2012-0047 grants. M. S. gratefully
acknowledges the support of TÁMOP 4.2.4.A/2-11-1-2012-0001
‘National Excellence Program’. The projects are supported by the
European Union and the State of Hungary, and co-financed by the
European Social Fund.
Please cite this article in press as: M. Sipiczki, et al., The catalytic epoxidation of 2-cyclohexen-1-one over uncalcined layered double
hydroxides using various solvents, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.031